In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array. Straightforward examples of an SLM are an amplitude SLM, where the pixels with the value ‘0’ block the light, and the pixels with the value ‘1’ transmit or reflect it, and a phase SLM, where the information bits ‘0’ and ‘1’ (or vice versa) are expressed by a phase shift of ‘0’ and ‘π’, respectively.
WO 2005/109410 discloses a so-called counter-propagating collinear holographic storage system. The object beam and the reference beam propagate in opposite directions and overlap in the holographic storage medium. In order to generate the counter-propagating beams the reference beam is transmitted through the holographic storage medium and impinges on a reflection type SLM, which generates the object beam by imprinting a data page onto the transmitted reference beam.
A schematic illustration of a counter-propagating system is depicted in FIG. 1. A light source 9 is used for generating the reference beam 2. The light source 9 may likewise generate the object beam, though this is not shown in the schematic illustration. The object beam 1 and the reference beam 2 overlap within the holographic storage medium 5, which is placed in or near to the Fourier plane 10 between two objective lenses 3, 4. The intensity distribution of the reference beam 2 within the holographic storage medium 5 has typically a Gaussian-like shape, as shown in FIG. 2. As a consequence, the high frequency components of the object beam 1, which are farther away from the center of the focus point, interfere with reference beam components of low intensity. In contrast, the low-frequency components of the object beam 1, which are situated closer to and at the center of the object beam 1, interfere with reference beam components of high intensity. This means that the holograms are effectively low-pass filtered during the holographic storage process.